1. Field of the Invention
The present invention relates to a D/A converter and to an electron beam exposure apparatus. More specifically, the present invention relates to a D/A converter provided with a self-calibrating function and to a multi-column electron beam exposure apparatus equipped with the D/A converter and provided with multiple columns to perform an exposure process in parallel on a single wafer.
2. Description of the Prior Art
In recent years, electron beam exposure apparatuses are coming into use to form fine patterns in lithographic processes for manufacturing semiconductor devices and the like.
An electron beam exposure apparatus provides better resolution than a photolithography apparatus but also has a problem of lower exposure throughput than the photolithography apparatus. To deal with this problem, a multi-column electron beam exposure apparatus has been proposed which has exposure throughput improved by being provided with multiple columns (each called a column cell) to form patterns on a resist by irradiating the resist with electron beams. Though each of the column cells is equivalent to a column in a single-column electron beam exposure apparatus, the multi-column electron beam exposure apparatus can multiply the exposure throughput by the number of the columns by performing the processes in parallel at the same time.
As a related technique, Japanese Patent Application Publication No. Hei 11-329322 discloses a multi-column electron beam exposure apparatus configured to expose identical patterns in parallel at the same time onto a single wafer while correcting pattern data according to displacement of an optical axis in each of the columns.
Meanwhile, in order to improve the exposure throughput, a mask is provided with either a variable rectangular aperture or multiple mask patterns, and a pattern is selected by beam deflection and thus is transferred onto a wafer by exposure. There is disclosed an electron beam exposure apparatus configured to perform character projection by using multiple mask patterns as described above. In the character projection, a pattern is transferred onto a sample surface as described below. Specifically, a certain pattern region selected by beam deflection from multiple patterns arranged on the mask is irradiated with a beam, and a beam cross section is shaped into the pattern shape. Then, the beam passed through the mask is deflected back by a deflector located at a later stage, reduced to a certain reduction ratio determined by an electron optical system, and transferred to a desired position on a sample by a sample surface positioning deflector.
Such an electron beam exposure apparatus deflects the beam based on exposure data provided in advance to select a character projection pattern or to irradiate a predetermined position on the wafer with the beam. The exposure data is digitalized and transmitted to deflection units of the respective columns and converted into analog data through D/A conversion, whereby signals are applied to deflection electrodes.
The multi-column electron beam apparatus requires, for each beam, approximately 40 channels of high-resolution (multi-bit) high-speed D/A converters for the purposes of mask selection and beam positioning on the wafer. Accordingly, an exposure apparatus configured to perform four-beam parallel processing requires approximately 160 channels.
Assuming that minimum drawing position accuracy is 0.5 nm, the electron beam exposure apparatus is required to have resolution of 4×105 or more in order to cover a deflecting region of 200×200 μm2, and accordingly needs a D/A converter having resolution of about 19 bits as well as stability.
In order to achieve a high-speed D/A converter, Japanese Patent Application Publication No. Sho 59-186416 discloses a D/A converter with a segment structure. Also, a current switch D/A converter using a bipolar transistor for higher speed is commercially available. However, these D/A converters cannot be expected to achieve accuracy of 17 bits or more due to base current variation or collector-emitter voltage variation.
Meanwhile, in order to achieve high accuracy, the D/A converter generally adopts a configuration to obtain output bit currents by using a current mirror while making reference to a highly accurate reference current. However, this configuration may only achieve accuracy around 16 bits because variation of around ten and several parts per million is assumed during monolithic manufacturing.
Moreover, trimming using a laser or a fuse is generally performed to assure linearity. However, this trimming is high in cost and low in accuracy. In addition, the linearity cannot be assured over time after the D/A converter is mounted on the apparatus, because the D/A converter varies over time.